Exists support lookup_name parameter.

Author: WaVEV

django_mongodb/expressions.py

@@ -172,9 +172,9 @@ def subquery(self, compiler, connection, lookup_name=None):
     return self.query.as_mql(compiler, connection, lookup_name=lookup_name)
 
 
-def exists(self, compiler, connection):
+def exists(self, compiler, connection, lookup_name=None):
     try:
-        lhs_mql = subquery(self, compiler, connection)
+        lhs_mql = subquery(self, compiler, connection, lookup_name=lookup_name)
     except EmptyResultSet:
         return Value(False).as_mql(compiler, connection)
     return connection.mongo_operators["isnull"](lhs_mql, False)